Scientists call it “global dimming,” a little-known trend that may be making the world darker than it used to be.

Thanks to thicker clouds and growing air pollution, much of Earth’s surface is receiving about 15 percent less sunlight than it did 50 years ago, according to Michael Roderick, a climate researcher at Australian National University in Canberra.

Researchers say global dimming, also known as solar dimming, partly offsets the global warming that most scientists agree is produced by “greenhouse gases,” including auto exhaust and emissions from coal-burning power plants.

The solar-dimming effect is “about half as large as the greenhouse-gas warming,” says James Hansen, director of NASA’s Goddard Institute for Space Studies in New York. In global warming, gases in the atmosphere, such as carbon dioxide, trap some of the sun’s heat and keep it from radiating back out to space, thereby raising Earth’s temperature. Clouds and air pollution, on the other hand, block a portion of the heat energy coming from the sun, just as it’s cooler sitting under a beach umbrella than under a bright sky.

Although global warming has been widely accepted, global dimming remains controversial. The theory has been advanced in recent years by a few researchers who measure the decline of solar radiation at hundreds of sites globally.

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